In typical PTCA procedures, a guiding catheter is percutaneously introduced into the cardiovascular system of a patient and advanced through the aorta until the distal end is in the ostium of the desired coronary artery. Using fluoroscopy, a guide wire is then advanced through the guiding catheter and across the site to be treated in the coronary artery. An over the wire (OTW) balloon catheter is advanced over the guide wire to the treatment site. The balloon is then expanded to reopen the artery. The OTW catheter may have a guide wire lumen which is as long as the catheter or it may be a rapid exchange catheter wherein the guide wire lumen is substantially shorter than the catheter. Alternatively, a fixed wire balloon catheter could be used. This device features a guide wire which is affixed to the catheter and cannot be removed.
To help prevent arterial closure, repair dissection, or prevent restenosis, a physician can implant an intravascular prosthesis, or a stent, for maintaining vascular patency inside the artery at the lesion. The stent may either be a self-expanding stent, a balloon assisted expandable stent or a balloon expandable stent. For the latter type, the stent is often delivered on a balloon and the balloon is used to the expand the stent. The self-expanding stents may be made of shape memory materials such as nitinol or constructed of regular metals but of a design which exhibits self expansion characteristics.
In certain known stent delivery catheters, a stent and an optional balloon are positioned at the distal end of the catheter, around a core lumen. The stent and balloon are held down and covered by a sheath or sleeve. When the distal portion is in its desired location of the targeted vessel the sheath or sleeve is retracted to expose the stent. After the sheath is removed, the stent is free to self-expand or be expanded with a balloon.
In a coronary stent deployment system which utilizes a retractable sheath one problem which is encountered is the interaction of the sheath and the stent upon retraction of the sheath. Typically, as the sheath slides off of the stent, the stent is subjected to potential marring by the sheath. While this problem can be minimized by making the sheath of soft materials, such materials are often unsuitable for use with a self-expanding stent where prolonged storage results in creep deformation of the inner sheath.
It is desirable to provide a medical device delivery system which provides a protective, non-marring inner sheath for the medical device and is capable of retaining the medical device for brief periods of time and which further has an additional outer sheath over the inner sheath which is capable of retaining the medical device for lengthy periods of time, thereby allowing the device to have a suitable shelf life.